Smell and Taste

Sharks are famous for their remarkably acute sense of smell. In contrast,
sharks are widely regarded as having little or no sense of taste. The first
of these perceptions is well documented and thus warranted, the second is
less clearly so. The scent organs of the White Shark are paired structures
located in capsules about mid-way along the undersurface of the snout, each
covered by a relatively simple flap of skin. The taste organs of the Great
White are finger-like buds scattered over the lining of the mouth and
pharynx, with the greatest density occurring on the soft tissue just behind
the teeth. Although most people think of smell and taste as separate senses,
they are actually different gradations of the same sensory experience.

Both olfaction (smell) and gustation (taste) depend upon a dissolved
sample of chemical compound fitting into a receptor cell, rather like a key
fits into a lock. When a chemical fits into a receptor, an electrical change
is induced in the cell that is transmitted via the nervous system to the
brain, where the stimulus is interpreted. As different chemicals have
differently shaped molecules, a variety of shape-specific receptors is
required. The 'tightness' of the fit between chemical and receptor dictates
the intensity of sensation. The major difference between the 'two'
sensations is quantity of chemical sampled: smell is an imprecise sampling
of small quantities of chemical carried by a transporting medium some
distance from its source; taste is a detailed sampling of a large quantity
of chemical in direct contact with chemical receptors. But since both smell
and taste require actual contact between a shape-specific receptor cell and
a dissolved chemical sample, they are merely different degrees of the same
sense, termed chemoreception.

Because a shark's olfactory organs are blind sacs not in any way
connected to the respiratory passageways, their external openings are termed
nares rather than nostrils. While many bottom-dwelling elasmobranchs have
nares adorned with elaborate flaps (often with grooves connecting the nares
to the mouth) that can control water-movement over the olfactory organs,
those of the White Shark have small, relatively simple flaps. As a Great
White swims, scent-bearing water flows into and out of each olfactory
capsule in an S-shaped pattern. The olfactory organ itself is roughly
spherical and composed of a series of closely spaced, parallel lamellae
(plates) studded with chemoreceptors. This arrangement maximizes the number
of receptors that can be packed into the smallest possible space, greatly
increasing sensitivity. The olfactory sensitivity of sharks in general is
nearly legendary, fostered by countless wide-eyed stories of these predators
following a trail of blood a quarter-mile (four-tenths of a kilometre) or
more to its source. Laboratory tests of shark olfactory acuity have revealed
that even these anecdotal tales pale in comparison to carefully measured
reality. Experiments on isolated olfactory lamellae of certain skates
(family Rajidae) have revealed astonishingly low threshholds to chemical
stimuli — responding to concentrations as low as 10-14 moles per litre of
water for the amino acid serine (or about 1 molecule of serine in 1015
molecules of water). In terms of relative volume, this is comparable to
detecting a golf ball in Loch Ness.

No one has yet measured the olfactory acuity of the White Shark, but
there is good neurological and behavioral evidence to suggest that this
species' scent tracking ability is exceptional. A 1996 paper by
neurophysiologists Leo Demski and R. Glen Northcutt examined the brain and
cranial nerves of the Great White. Demski and Northcutt found that 14
percent of the White Shark's total brain mass is composed of the olfactory
bulbs. This is over 4.5 times the proportion Northcutt had previously found
in a typical skate and twice that in the Scalloped Hammerhead (which has
huge, sausage-shaped olfactory bulbs). In fact, Demski and Northcutt point
out that the White Shark has the largest olfactory bulbs relative to brain
mass of any cartilaginous fish examined to date. Since so much of the Great
White's brain is dedicated to olfaction, it seems reasonable to conclude
that scent is highly important to this species. Shark behaviorist Rocky
Strong has speculated that the strong odors that characterize seal and sea
lion colonies may offer White Sharks a wealth of olfactory stimuli, enabling
these predators to locate concentrations of pinnipeds. A number of field and
aquarium observations indicate that pheromones (hormonal secretion used to
communicate within species) may play an important role in shark reproduction
by signalling a female's readiness to mate. In a 1983 paper, Demski and
Northcutt reported their discovery of a cranial nerve in the common Goldfish
(Carassius auratus) that is apparently dedicated to the detection of
pheromones; in their 1996 paper, Demski and Northcutt confirmed that this
nerve is both present and well-developed in the White Shark. A dedicated
pheromone-detection system may enable sexually receptive but widely
separated White Sharks to find one another in the vastness of the World
Ocean.

How, exactly, sharks track odors in the open sea has long been a mystery.
Given their remarkable olfactory acuity and apparent zig-zag hunting
behavior, it had traditionally been assumed that sharks could actually
compare the relative intensity of scent received by each nare. It was
believed that, by continually adjusting their course according to whichever
nare received the strongest whiff, a hunting shark could quickly locate the
source of any attractive odor. Some workers pointed out that a major problem
with this rather complicated scenario is that seawater dissipates chemicals
very rapidly. It was therefore supposed that, at any significant distance
from the source of a water-borne chemical, there would be only a few
molecules of attractant for every ten or hundred billion molecules of water.
Under such conditions — and despite its astonishing olfactory acuity — it
seemed extremely unlikely that a shark could detect any concentration
gradient that might exist in the relatively short distance between its two
nares. Yet in a 1985 paper, Peter Johnsen and John Teeter demonstrated that
Bonnethead Sharks (Sphyrna tiburo) actually could detect and respond to
concentration gradients between the left and right nares. The Bonnethead is
a small species of hammerhead, with a 'hammer' that is only slightly
expanded laterally. But it has not yet been demonstrated that non-hammerheaded
sharks — such as the Great White — can detect which nare receives the
stronger whiff.

It is more likely that the directional mechanism of scent tracking in
most sharks is refreshingly simple. When a point source releases chemical
compounds into the ocean, the prevailing currents establish a rapidly
dissipating odor corridor. A shark's lateral line system enables it to
detect subtle water movements. Therefore, when a shark's acute olfactory
system detects an attractive chemical, all it needs to do is turn into the
current. Sooner or later, this will bring a shark to the source of the odor.
But the marine environment is huge and concentrated food sources are often
few and far between, especially in the open ocean. Volatile chemicals — such
as the gases that might be liberated from a decaying whale carcass — are
dispersed through air much more quickly than through water. An intriguing
1994 paper by Russian sensory biologists S.V. Savel'ev and V.P. Cherinkov
suggests that at least one pelagic shark, the Oceanic Whitetip (Carcharhinus
longimanus) is able to employ aerial olfaction in the search for food. They
found that the close-packed, collagen-strengthened olfactory lamellae of the
Oceanic Whitetip enable this species to trap and detect surface bubbles that
may carry airborne scents. Savel'ev and Cherinkov found that the
loosely-packed, floppy lamellae of the Spiny Dogfish could not trap bubbles.
The authors proposed that, by holding its snout tip above the surface, an
Oceanic Whitetip may be able to detect airborne scents and locate distant
food sources more quickly than many potential competitors.

One of the most curious behaviors of the White Shark is a habit of
raising its head above the water surface by up to 3 feet (1 metre). As
visually-biased creatures, people had long assumed that a Great White
performing this un-fishy act was raising its eyes above the surface to
visually inspect potential meals, such as hauled out pinnipeds (seals and
sea lions) or humans on boats. This is a chilling thought. But, given the
profound refractive shift between water and air, I doubt that the White
Shark's lens-moving muscles could accommodate strongly enough to enable the
animal to see clearly. Yet Demski and Northcutt's study of brain
organization in the White Shark revealed that this species has particularly
well developed olfactory bulbs. Perhaps — like the Oceanic Whitetip — the
Great White can detect airborne scents, enabling it to locate large
concentrations of odoriferous food, such as a floating whale carcass or
pinniped rookery. Since wind is slowed by friction with the ocean surface,
air movement is slower at the air-water interface than above it. By raising
its head several feet out of the water, a White Shark may be better able to
detect airborne scents that could indicate a rich source of food in the
otherwise featureless expanse of the open ocean.

To what extent the Great White relies on its sense of taste is also a
matter of conjecture. There are a few, scattered reports of other sharks
apparently rejecting food on the basis of taste, but this has not been
tested. In a 1980 paper, biologists Jack Ames and G. Victor Morejohn
reported a perplexing mystery: at least 59 (9%) of all Sea Otter (Enhydra
lutris) carcasses washed up on California beaches between 1968 and 1979 had
clear evidence of having been bitten by Great Whites: fragments of White
Shark teeth embedded in the open wounds. The phenomenon of shark bitten Sea
Otter carcasses had been known for decades. Yet no Sea Otter had ever turned
up among the stomach contents of White Sharks. Why not? In his excellent
1976 book, artist-author Richard Ellis speculated that, since Sea Otters are
members of the weasel family (Mustelidae) — a group which includes skunks
and many other strong-smelling relatives — perhaps the Otters emitted an
odor that is distasteful to White Sharks. This certainly seemed reasonable
at the time. But new evidence suggests a more intriguing solution to this
mystery.